Heat engine is a device by which a system is made to undergo a ...X... process that result in conversion of ...Y... into work.
Here, X and Y refer to

 

Heat engine is device by which a system is made to undergo cyclic process that result in conversion of heat into work.

When gas (system) in heat engine undergoes process 1$\to$  2$\to$   3$\to$   4 $\to$  1, then work done by gas = area enclosed by figure formed by joining 1, 2 , 3 , 4. Work is positive if arrows move clockwise.